This invention relates to radio receivers and particularly but not exclusively to such receivers forming part of the transceiver of a portable radio telephone or handset, for communication with a base station in a radio telephone system. The invention also relates to a method of operation of such radio receivers. The invention also relates, particularly but not exclusively, to handsets for communication with base stations in a digital cellular radio telephone system employing transmission by a plurality of carrier frequencies in frames, such as TDMA frames, each consisting of a predetermined number of time slots.
Systems operating under the same protocol can cause problems if their spheres of influence overlap. This is because signals from a user on one system may occur at the same time as a signal from a user of the other system. When the two systems are unsynchronised, this pattern of possible interference is not fixed, instead, as the timing of one system will drift relative to the other, the interference pattern will differ with time. A pair of communication channels utilised on respective systems can `slide` in and out of interference with one another over a period of time. Such interference is known as a `sliding collision`.
FIG. 1 is an illustration of a sliding collision. In FIG. 1a, bursts from system 1 and system 2 do not overlap. There is, therefore, no interference i.e. no `collision`. As the bursts drift, one relative to the other, the two bursts come closer (FIG. 1b) and begin to interfere 3 (FIG. 1c). As time passes, interference progressively increases (FIG. 1d).
The sliding collision illustrated in FIG. 1 results in the burst of system 1 losing data at the back end of the burst, while the burst of system 2 loses information from the front of the burst. This type of corruption of a signal has the potential to be very damaging to signal quality and should, therefore, be detected as quickly as possible so that a new channel can be selected. Sliding collisions are difficult to avoid as quickly as might be desirable as, in order to prevent thrashing i.e. frequent or continual hopping of channels, there is normally a delay or hysteresis before a determination to change (handover) channels is made for any detected errors. This prevents temporary deteriorations in channel quality from precipitating channel handover. Such temporary channel deteriorations would typically be caused by fast fades.